My researches on the visual organs of the Cambrian trilobites are founded on the Angelinian Collection in the Swedish State Museum together with collections of foreign species, but also largely on the waste European and American literature, though we have to deplore the often occurring inexactitude of the figures, especially in the older works, and constructed or schematized figures in some of the newer ones, which give a quite false notion of the structure. There is no lack of figures to show how it ought to be, according to preconceived notions and, on the other hand, a great scarcity of representations, to show how it really is. In spite of all this there is a sufficiently great number of well established facts to demonstrate the organization of the Cambrian genera. The trilobites of this division may be called blind only in so far as they have no eyes on the upper surface of the head, but they may have been provided with visual organs, though more imperfect, on the hypostoma as really seems to have been the case with some of them. According to the structure of the head shield the blind trilobites may be subdivided into the following well characterized groups. These are: I. Without facial ridge: 1) The Archæan Trilobites. II. With facial ridge: 1) The Olenellidæ. 2) The Olenidæ and related. =I. Blind trilobites without facial ridge (= »eye lobe»).= =Group 1. The Archæan Trilobites.= In these the head shield is in one piece without any facial suture and facial ridge, and without the least trace of anything that might be called a visual organ and they must consequently be considered as totally blind. In contradistinction to the following groups, excepting the oldest Olenellidæ, the head consists of only three parts, 1) glabella, and 2-3) the two fixed cheeks. These genera range from the oldest zone in which hitherto trilobites have been found, that of Olenellus (Holmia) Kjerulfi, to the zone of Paradoxides Forchhammeri, and some, as Agnostus, even continue as high in the Lower Silurian series as in the Brachiopod schists. Beside Agnostus the other genera are Conocoryphe (seven species in the Swedish Cambrium), Toxotis, Ctenocephalus (?), Elyx, Aneuacanthus, Conophrys and Microdiscus. »Harpides» breviceps ANG., also belongs here. Anopocare of ANGELIN should also be regarded as one of this group. But it cannot be retained any longer because it is founded on two other, well known forms, being, according to LINNARSSON, Peltura scarabæoides (pl. 27 fig. 1, a in Pal. Scand.) and young specimens of Sphærophthalmus alatus (ibid. figs. 1 & 2).